In the field of transportation, the continued development of battery technology enabled intensified use of electrically powered vehicles. It is estimated that the use of electrically powered vehicles, also referred to as “electric vehicles”, will continue to rise in the near future. Besides vehicles that are entirely operated using electric power, hybrid vehicles are available using various drivetrain setups, such as parallel- and series-type hybrid vehicles. For example, hybrid vehicles are commercially available, which are in part operated by muscle power of an operator, and which also comprise an electric motor, e.g., for support of the operator, to minimize fatigue and to extend the range. Such drive concepts are in particular used in bicycles, tricycles, quadracycles, boats, airplanes or helicopters, i.e., virtually in any type of vehicle.
In the recent past, series-type hybrid drives have been made available, where an operator provides input muscle power by using one or two foot-pedals, levers, or handles. The provided input muscle power is converted into electric energy using a generator, which is mechanically coupled to the respective pedal, lever, or handle. The electric energy is then fed to an electric motor to drive the vehicle, e.g., together with electric energy from a battery in case some support, also known as “power assist”, is required. Accordingly, vehicles using this setup are also referred to as having an “electric transmission”, since there is no connection between pedal and wheel that could convey mechanical propulsive power. These vehicles are similar to the common “Pedelecs”, but the mechanical setup of such vehicles is much simpler and thus cheaper than the typical setup of a Pedelec, in particular since no chain, belt, or transmission shaft and no elaborate mechanical or hydraulical gear shift mechanism is necessary. In addition, hybrid vehicles with an electric transmission can be configured in a flexible way, e.g., to match the use of the respective operator.
A particular challenge with series-type, muscle-operated hybrid electric vehicles, however, is given in that an operator typically expects the mechanical interaction, i.e., the “feel” or “feedback” of the drive system to be similar to that of a known corresponding vehicle having a mechanical drivetrain. For example, in a case of a hybrid electric bicycle having pedals, an operator typically expects the pedals to respond like the pedals of a common “mechanical” bicycle, including the usual resistance torque of the pedal due to the inertia of the bicycle and its chain/wheel drive.
In a series-type hybrid electric vehicle, the mass of the pedals, the generator coupled to them and an optional transmission in between usually is negligible, e.g. in comparison to the total mass of the vehicle and the operator. Therefore, this mass does not give rise to any significant resistance torque. Further, some resistance torque is generated by “dissipative” or “damping” effects such as mechanical friction, eddy currents and hysteresis losses. The order of magnitude of the torque due to these effects can be e.g. 1-3 Nm, which is rather low. Also, the operator may experience an electrical resistance torque due to the power generation in the generator. However, this torque, which, in most electrical machines, is proportional to the output current of the generator, can also be relatively low, depending on the type of generator and how it is operated. This leads to an unexpected “feel” of the drive system during use. The unexpected feel or behavior of such vehicle may in turn be conceived by an operator as not particularly ergonomic. In addition, in the case of a foot-operated series-type electric bicycle, a lack of enough pedal resistance torque can possibly lead to dangerous situations. For example, upon starting to pedal, a lack of pedal resistance torque may cause the operator to lose balance on the vehicle or even slide off the pedal, since the behavior is not as it would be expected from a common bicycle having a traditional bicycle drive train.
In the prior art, the problem of an unexpected behavior of series-type hybrid electric vehicles was addressed, e.g., in WO 00/059773 A2 of the present inventor. The latter document in particular addresses the situation upon starting to pedal and improves the behavior significantly. US 2009/0095552 A1 describes a further approach to provide a largely “natural” behavior of a series-type hybrid vehicle, comparable to a mechanically driven vehicle. Here, the speed of the pedal crank is related to the travel velocity in a way that is comprehensible to a user of the vehicle. The system of US 2009/0095552 A1 comprises a braking unit, which opposes the rotation of the pedal crank. The braking unit may, e.g., comprise a flywheel mass. The pedal crank of this reference is not primarily used for generating energy to propel the vehicle, but rather for controlling the travel velocity of the vehicle.
Certainly, carrying along a flywheel mass on an hybrid electric vehicle leads to a reduced efficiency due to unnecessary friction and increased weight. But even without using a flywheel mass, the direct coupling of the pedal crank with the vehicle speed according to the prior art may be disadvantageous, since due to the characteristic pedaling of a human, inevitable variations in the provided pedaling speed causes variations in the vehicle speed, which in turn may result in poor vehicle handling, such as in particular reduced traction control on hard and slick surfaces, e.g., during inclement weather conditions.
Accordingly, a pedal drive system is needed that provides an improved haptic feel and feedback to a user/operator, while avoiding one or more disadvantages of the prior art.